Adding Team property to MessageEvent

Resolves #89

Author: Simon Oxtoby

SlackNet/Events/Messages/MessageEvent.cs

@@ -13,6 +13,7 @@ public class MessageEvent : Event
         public string User { get; set; }
         public string Text { get; set; }
         public string Ts { get; set; }
+        public string Team { get; set; }
         [JsonIgnore]
         public DateTime Timestamp => Ts.ToDateTime().GetValueOrDefault();
         public string ThreadTs { get; set; }